<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title>JavaScript变量之undefined类型</title>
    </head>
    <body>
        <h3>JavaScript变量之undefined类型</h3>
        <hr />
        <script>
            //声明变量msg
            var msg;
            //在alert()方法中使用变量msg
            alert(msg);
        </script>
    </body>
</html>